The present invention relates to a method and apparatus for manufacturing a sinter by sintering a laminate including a layer containing a ceramic powder or a metal powder and an organic layer such as a binder or a plasticizer, and other techniques related thereto.
In recent years, a multilayer ceramic component, including ceramic layers and conductive layers layered on one another, has been downsized and the performance thereof has been improved. A multilayer ceramic component will now be described with respect to a multilayer ceramic capacitor, which is one type of a multilayer ceramic component. For a multilayer ceramic capacitor, which has been used as a main component of a mobile station (hand held terminal) in a mobile communication system, such as a portable telephone, there is a demand for reducing the size thereof while increasing the capacity thereof, i.e., improving two contradicting characteristics thereof. To meet such a demand, attempts have been made in the art to reduce the thickness of a dielectric layer, increasing the number of layers, and increasing the dielectric constant thereof by developing a new material therefor. However, since there is a limit on the reduction of the thickness of a dielectric layer or the increase in the dielectric constant thereof by developing a new material therefor, the area of a multilayer ceramic capacitor has been increasing each year in order to increase the capacity thereof.
A multilayer ceramic capacitor is typically manufactured as follows. First, a dielectric material powder, a binder, a plasticizer, a dispersant, a solvent, etc., are mixed together and stirred to produce a dielectric slurry having an appropriate viscosity. Typically, barium titanate is used for the dielectric material, an acrylic resin for the binder, an ester-based plasticizer such as dibutyl phthalate for the plasticizer, an anionic surfactant such as carboxylate for the dispersant, and an ester such as butyl acetate, an alcohol, an ether or a hydrocarbon for the solvent. On the other hand, an internal electrode paste is produced by mixing a metal powder such as nickel and an organic substance together into a paste.
Then, a dielectric film is produced by directly screen-printing the dielectric slurry using a print laminator, or the like. Then, an internal electrode pattern is produced similarly by directly screen-printing the internal electrode paste using a different screen. The screen printing process for a dielectric film and an internal electrode pattern as described above is repeated for a desired number of times so as to produce a green sheet including electrodes and dielectric films alternating with each other. After the lamination step, the green sheet is cut into pieces according to the size of each printed chip, thereby obtaining a green laminate. Then, the green laminate is placed into a case in preparation for baking, and the process proceeds to a degreasing step and a baking step. The degreasing step refers to a step of removing organic substances contained in the green laminate such as a binder, a plasticizer, a solvent, etc. The baking step refers to a step of sintering through a reaction between ceramic grains. After the completion of the baking step, an external electrode, etc., are formed, after optional steps such as polishing the side surface, etc., in order to connect the internal electrodes to each other and at the same time to extract the terminals to the outside. Depending on the structure of the ceramic laminate component, a base electrode, an intermediate electrode, etc., may be formed.
The degreasing step takes a large portion of the total amount of time for the manufacture of the multilayer ceramic capacitor. As described above, in the degreasing step, organic substances such as a binder and a plasticizer, which are used for maintaining the shape of the green sheet, are decomposed and removed. Normally, the degreasing step is performed in the air. In order to increase the manufacturing efficiency, it is preferred to perform the degreasing step while rapidly increasing and decreasing the temperature of the furnace. However, if the furnace is rapidly heated, the organic substances such as a binder and a plasticizer are rapidly evaporated and decomposed, which may induce structural defects such as delamination or a crack. Such defects significantly influence the quality of the product. In view of this, the green laminate is presently degreased by slowly increasing and decreasing the temperature through a temperature range of 150 to 300xc2x0 C. over a time period of 20 hours to several days. In recent years, the amount of time for the degreasing step has been increasing due to an increase in the size (particularly, the area) of a chip along with an increase in the capacitance of a capacitor. Moreover, while degreasing is done in order to remove organic substances from a green laminate, as described above, the green laminate contains a mixture of various organic substances, including a binder, a plasticizer, a dispersant, a solvent, etc., and these organic substances have different evaporation temperatures and different decomposition temperatures. Therefore, the degreasing step requires a very precise temperature control and process control.
As described above, the degreasing step requires a very precise temperature control and process control, and the amount of time for the degreasing step has been increasing due to an increase in the size and capacitance of a capacitor. With the conventional thermal decomposition/removal method performed in the air, it is very difficult to shorten the process time by speeding the degreasing step. On the other hand, many problems occur if the baking step is performed without sufficiently removing the organic substances such as a binder. Particularly, if there remains an amount of plasticizer as a result of failing to sufficiently remove it during the degreasing step, benzene rings in the plasticizer react in the baking step to produce a graphite-like substance, and the graphite-like substance causes various defects in the multilayer ceramic component. First, the graphite-like to substance has an expansion coefficient different from those of ceramics, an internal electrode material, etc., whereby it is likely to induce structural defects such as delamination or a crack as described above. Moreover, the graphite-like substance has a xcfx80 electron and thus a high conductivity, whereby it may cause a leak current between the internal electrodes. As described above, if the plasticizer is not removed in the degreasing step, the manufacturing yield decreases, and the performance of the multilayer ceramic component also decreases. Therefore, it has been unavoidable to perform a time-consuming degreasing step in the prior art.
Moreover, with the conventional thermal decomposition/removal method performed in the air, an organic substance may be present in a gas that is discharged during the de-binder step, thereby also presenting an environmental problem. Therefore, it has been necessary to completely decompose and incinerate the organic substance or remove the organic substance by adsorption, etc., thus presenting a cost problem.
Methods for degreasing ceramics by using a supercritical fluid have been proposed in the art (e.g., Chemical Engineering, 1986 May issue, pp. 46-49, Shozaburo Saito, xe2x80x9cChourinkairyutai No Kagaku to Gijutsu (Science And Technology Of Supercritical Fluid)xe2x80x9d, Sankyo Business). Generally, a supercritical fluid has a high dissolving ability. The degreasing of a ceramics compact can be done in a very short time by removing a binder using a supercritical fluid. With this method, however, the function of maintaining the shape of a green laminate deteriorates when removing a plasticizer and a binder from the green laminate, thereby deteriorating the precision in the shape of the product.
The present invention is based on a discovery that the total amount of time for the degreasing step can be reduced by quickly removing only a plasticizer among various organic substances that are used in a green sheet, and an object of the present invention is to provide a method and apparatus for manufacturing a multilayer ceramic structure in which the manufacturing efficiency is improved while maintaining the shape of the green laminate at a high precision, by utilizing the characteristics of a supercritical fluid.
A first method for manufacturing a sinter of the present invention includes the steps of: (a) layering an internal electrode layer containing at least a conductive material powder and a dielectric layer containing a ceramic material powder, a binder and a plasticizer on each other, so as to produce a green laminate; (b) contacting the green laminate with a supercritical or subcritical fluid so as to extract/remove the plasticizer in the green laminate; (c) after the step (b), decomposing and removing the binder in the green laminate; and (d) after the step (c), sintering the green laminate.
With the conventional degreasing step in which the plasticizer and the binder are removed from the green laminate through thermal decomposition, structural defects occur in the green laminate if the temperature is increased rapidly, and the process time becomes undesirably long if the temperature is increased gradually so as to prevent the occurrence of the structural defects. In contrast, with the method of the present invention, the plasticizer is selectively extracted/removed from the green laminate by using the supercritical or subcritical fluid before the de-binder step, thereby suppressing the formation of a graphite-like substance even if the temperature is increased rapidly in the subsequent de-binder step. Therefore, it is possible to increase the manufacturing efficiency without reducing the manufacturing yield or the performance of the product due to the occurrence of structural defects in the green laminate and thus in the sinter.
In the step (a), at least one resin selected from a butyral resin, an acrylic resin, a polypropylene and a polyethylene may be used as the binder.
In the step (a), at least one substance selected from an ester, stearic acid, stearyl alcohol and a paraffin may be used as the plasticizer. Particularly, it is preferred to use a phthalate ester.
Preferably, in the step (a), a paraffin that is present in a form of a solid during the step (b) is used as the plasticizer.
Preferably, in the step (b), at least one substance selected from carbon dioxide, a hydrocarbon and a polyhalogenated hydrocarbon is used as the supercritical or subcritical fluid.
Preferably, in the step (b), carbon dioxide is used as the supercritical or subcritical fluid, and the temperature of carbon dioxide is maintained in a range from room temperature to 50xc2x0 C., or in a range from 140xc2x0 C. to a temperature used in the step (c).
In the step (b), at least one substance selected from an alcohol, a ketone and a hydrocarbon may be mixed in the supercritical or subcritical fluid as an entrainer (extraction assistant).
In the step (b), a pressure of the fluid containing the plasticizer that has been extracted/removed from the green laminate may be reduced so as to turn the fluid into a gaseous state, thereby separating the fluid and the plasticizer from each other to collect the plasticizer. In this way, it is possible to save the time and cost for disposing of the plasticizer.
In the step (a), at least one metal selected from Pt, Pd and Ni may be used as the conductive material powder.
In the step (b), a pressure of the supercritical or subcritical fluid may be changed with time. In this way, it is possible to further improve the plasticizer extraction/removal efficiency.
In the step (b), an ultrasonic vibration may be applied to the fluid. Also in this way, it is possible to further improve the plasticizer extraction/removal efficiency.
The method may further include the step of subjecting the green laminate to a heat treatment at 250xc2x0 C. or more in a vacuum or a gas, between the step (a) and the step (b). In this way, it is possible to more effectively suppress the occurrence of structural defects in the green laminate and thus in the sinter.
In the step (b), the green laminate may be pressurized by using a pressurization medium, and then the pressurization medium may be substituted by the supercritical or subcritical fluid. In this way, it is possible to more effectively suppress the occurrence of structural defects in the green laminate and thus in the sinter.
Preferably, an inert gas is used as the pressurization medium, and at least one substance selected from carbon dioxide, a hydrocarbon and a polyhalogenated hydrocarbon is used as the supercritical or subcritical fluid.
In the step (b), the fluid may be turned into a supercritical or subcritical state by rapidly pressurizing the fluid at a rate of 1 MPa/min or more. In this way, it is possible to more effectively suppress the occurrence of structural defects in the green laminate and thus in the sinter.
Preferably, the method further includes the step of evaluating a concentration distribution of the plasticizer in the green laminate by using a microscopic laser Raman spectroscopy method, at any point after the step (b) and before the step (d).
More preferably, a relative concentration distribution is obtained by calculating a relative intensity by normalizing an intensity of a plasticizer-induced absorption band peak of the Raman spectrum with respect to an intensity of a ceramic-induced absorption band peak.
A second method for manufacturing a sinter of the present invention includes the steps of: (a) compacting a mixture of an inorganic substance and an organic substance so as to obtain a compact; (b) contacting the compact with a supercritical or subcritical fluid so as to extract/remove the organic substance in the compact; and (c) after the step (b), sintering the compact so as to obtain a sinter, wherein a material that is in a liquid state in the step (b) and in a solid state in the step (c) is used as the organic substance.
With this method, the organic substance has a fluidity in the step (a), whereby the compaction can be performed smoothly. On the other hand, the organic substance is a solid in the step (b), whereby the organic substance does not undergo a volumetric expansion. Therefore, it is possible to suppress the occurrence of structural defects in the compact upon completion of the step (b) and thus in the sinter.
A third method for manufacturing a sinter of the present invention includes the steps of: (a) compacting a mixture of an inorganic substance and an organic substance so as to obtain a compact; (b) after the step (a), subjecting the compact to a heat treatment in a vacuum or a gas so as to partially remove the organic substance; (c) after the step (b), contacting the compact with a supercritical or subcritical fluid so as to extract/remove the organic substance in the compact; and (d) after the step (c), sintering the compact so as to obtain a sinter.
With this method, the organic substance is partially removed in the step (b), thereby producing gaps in the compact. As a result, even if the fluid in the organic substance expands in the step (c), the expansion is buffered by the gaps, thereby suppressing the occurrence of structural defects in the compact and thus in the sinter.
A fourth method for manufacturing a sinter of the present invention includes the steps of: (a) compacting a mixture of an inorganic substance and an organic substance so as to obtain a compact; (b) after the step (a), pressurizing the compact by using a pressurization medium, and then substituting the pressurization medium by a supercritical or subcritical fluid so as to contact the compact with the supercritical or subcritical fluid, thereby extracting/removing the organic substance in the compact; and (c) after the step (b), sintering the compact so as to obtain a sinter.
Since the compact is already pressurized in the step (b), the fluid in the organic substance of the compact does not undergo a volumetric expansion, thereby suppressing the occurrence of structural defects in the compact and thus in the sinter.
Preferably, an inert gas is used as the pressurization medium, and at least one substance selected from carbon dioxide, a hydrocarbon and a polyhalogenated hydrocarbon is used as the supercritical or subcritical fluid.
A fifth method for manufacturing a sinter of the present invention includes the steps of: (a) compacting a mixture of an inorganic substance and an organic substance so as to obtain a compact; (b) contacting the compact with a supercritical or subcritical fluid so as to extract/remove the organic substance in the compact; and (c) after the step (b), sintering the compact so as to obtain a sinter, wherein in the step (b), the fluid is turned into a supercritical or subcritical state by rapidly pressurizing the fluid at a rate of 1 MPa/min or more.
With this method, a high pressure state is reached while the dissolution of the fluid in the organic substance is insufficient, and thus the organic substance undergoes little volumetric expansion, thereby suppressing the occurrence of structural defects in the compact and thus in the sinter.
A sixth method for manufacturing a sinter of the present invention includes the steps of: (a) compacting a mixture containing at least a conductive material powder or a ceramic material powder and an organic substance so as to obtain a porous material; (b) contacting the porous material with a supercritical or subcritical fluid so as to extract/remove the organic substance in the porous material; and (c) after the step (b), sintering the porous material at a high temperature so as to obtain a porous sinter.
Also with this method, it is possible to increase the manufacturing efficiency without deteriorating the quality due to the occurrence of structural defects in the compact and thus in the sinter.
Preferably, in the step (a), at least one substance selected from camphor, naphthalene and a paraffin is used as the organic substance.
An apparatus for manufacturing a sinter of the present invention includes: a pressure chamber capable of maintaining a fluid in a supercritical or subcritical state; a supply device for supplying the fluid; a temperature control device for controlling a temperature of the pressure chamber; a pressure control device for controlling a pressure of the pressure chamber; a depressurizing device for depressurizing the fluid discharged from the pressure chamber; and a plasticizer collecting device for turning the plasticizer into a liquid or a solid so as to separate and collect the plasticizer from the fluid in the depressurizing device.
In this way, it is possible to carry out the methods for manufacturing a laminate described above while saving the time and cost for disposing of the plasticizer.
The apparatus may further include a pressurizing device for pressurizing again the fluid that has been depressurized by the depressurizing device so as to turn the fluid into a supercritical or subcritical state. In this way, it is possible to save the time and cost for discharging the fluid into the atmospheric air.
The apparatus may further include a spectroscopic device for performing a spectroscopy of the supercritical or subcritical fluid. In this way, it is possible to accurately detect the completion of the plasticizer extraction/removal process through in situ observation.
A method for measuring a concentration of a plasticizer of the present invention is a method for measuring a concentration of a plasticizer in a compact that is obtained by compacting a mixture of an inorganic substance and an organic substance including the plasticizer and a binder, the method including the step of evaluating a concentration distribution of the plasticizer in the compact by using a microscopic laser Raman spectroscopy method.
With this method, it is possible to detect, with a high sensitivity, benzene rings and ester groups that are often not contained in a binder or an inorganic substance, by utilizing the characteristics of laser Raman light.
A relative concentration distribution may be obtained by calculating a relative intensity by normalizing an intensity of a plasticizer-induced absorption band peak of the Raman spectrum with respect to an intensity of an inorganic substance-induced absorption band peak.
An evaluation method of the present invention is an evaluation method for evaluating a volume of a substance in a supercritical or subcritical fluid, the substance being soluble in the fluid, the method including the steps of: (a) placing a transparent tubular member in which the substance has been introduced in a pressure chamber capable of maintaining the fluid in a supercritical or subcritical state; (b) measuring a length of a region of the tubular member over which the substance is present for each of various pressure values while changing a pressure in the pressure chamber to various values under a predetermined temperature by supplying the fluid into the pressure chamber; and (c) obtaining a pressure dependency of the volume of the substance in the fluid based on measurement results obtained in the step (b).
Where a substance, which is an object to be measured, is contained in a compact or a member, it is possible with this method to determine the conditions under which structural defects occur in the compact or the member due to a volumetric expansion of the substance. Thus, it is possible to appropriately determine the conditions for the process of manufacturing a component such as a multilayer ceramic component or capacitor.
An evaluation apparatus of the present invention includes: a pressure chamber capable of maintaining a fluid in a supercritical or subcritical state; a supply device for supplying the fluid; a temperature control device for controlling a temperature of the pressure chamber; a pressure control device for controlling a pressure of the pressure chamber; and a window member provided in a portion of the pressure chamber, the window member allowing light from an inside of the pressure chamber to be transmitted therethrough to an outside of the pressure chamber.
In this way, it is possible to accurately grasp the behavior of the object in a supercritical or subcritical fluid, thereby obtaining important indices for determining the manufacturing process conditions. Moreover, the evaluation apparatus can be used in the manufacturing process, in which case it is possible to perform a process using a supercritical or subcritical fluid while observing the inside of the apparatus in situ.
The evaluation apparatus may further include: a transparent tubular member placed in the pressure chamber wherein the substance can be introduced into the tubular member; and means for measuring a region of the tubular member over which the substance is present. In this way, it is possible to measure a change in the volume of the organic substance.